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<div class="section">
<div class="titlepage"><div><div><h2 class="title" style="clear: both">
<a name="usage"></a>Using libnm</h2></div></div></div>
<div class="simplesect">
<div class="titlepage"><div><div><h3 class="title">
<a name="id-1.2.3.2"></a>When to use libnm</h3></div></div></div>
<p>
          libnm is fairly simple to use from C. It's based on glib and GObject.
          If your project uses these already you'll find integration libnm with your
          project rather convenient. In fact, the <span class="command"><strong>nmcli</strong></span> tool shipped
          with NetworkManager is based on libnm.
        </p>
<p>
          libnm should be also the way to go if your project does something non-trivial
          with NetworkManager, such as manipulating the connection profiles.
          That is, if you're writing a specialized networking control tool or a desktop
          environment, libnm is probably the right choice. The popular desktop
          environments in fact all use libnm directly or with nm-applet and
          nm-connection-editor that are all based on libnm.
        </p>
<p>
          An alternative to use of libnm is the use of the
          <a class="ulink" href="https://networkmanager.dev/docs/api/latest/spec.html" target="_top">D-Bus API</a>,
          directly.  This gives you larger flexibility and reduces the overhead of linking
          with the libnm library. This makes sense if your task is simple and you have a good
          D-Bus library at your disposal. Activating a particular connection profile
          from a Python script is a good example of a task that is perfectly simple
          without using libnm.
        </p>
</div>
<div class="simplesect">
<div class="titlepage"><div><div><h3 class="title">
<a name="id-1.2.3.3"></a>How to use libnm</h3></div></div></div>
<p>
          You can use the libnm's C API directly. To do so, all libnm programs need to
          include <code class="filename">NetworkManager.h</code> that provides necessary definitions.
          The rest of the API is documented in the reference manual.
        </p>
<div class="informalexample">
  <table class="listing_frame" border="0" cellpadding="0" cellspacing="0">
    <tbody>
      <tr>
        <td class="listing_lines" align="right"><pre>1
2
3
4
5
6
7
8
9
10
11
12</pre></td>
        <td class="listing_code"><pre class="programlisting"><span class="cp">#include</span><span class="w"> </span><span class="cpf">&lt;glib.h&gt;</span>
<span class="cp">#include</span><span class="w"> </span><span class="cpf">&lt;NetworkManager.h&gt;</span>

<span class="kt">int</span>
<span class="nf">main</span><span class="w"> </span><span class="p">(</span><span class="kt">int</span><span class="w"> </span><span class="n">argc</span><span class="p">,</span><span class="w"> </span><span class="kt">char</span><span class="w"> </span><span class="o">*</span><span class="n">argv</span><span class="p">[])</span>
<span class="p">{</span>
<span class="w">	</span><span class="n">NMClient</span><span class="w"> </span><span class="o">*</span><span class="n">client</span><span class="p">;</span>

<span class="w">	</span><span class="n">client</span><span class="w"> </span><span class="o">=</span><span class="w"> </span><span class="n">nm_client_new</span><span class="w"> </span><span class="p">(</span><span class="nb">NULL</span><span class="p">,</span><span class="w"> </span><span class="nb">NULL</span><span class="p">);</span>
<span class="w">	</span><span class="k">if</span><span class="w"> </span><span class="p">(</span><span class="n">client</span><span class="p">)</span>
<span class="w">		</span><span class="n">g_print</span><span class="w"> </span><span class="p">(</span><span class="s">&quot;NetworkManager version: %s</span><span class="se">\n</span><span class="s">&quot;</span><span class="p">,</span><span class="w"> </span><span class="n">nm_client_get_version</span><span class="w"> </span><span class="p">(</span><span class="n">client</span><span class="p">));</span>
<span class="p">}</span></pre></td>
      </tr>
    </tbody>
  </table>
</div>

<p>
          Use <span class="command"><strong>pkg-config</strong></span> for <code class="varname">libnm</code> to discover the necessary
          compiler flags.
        </p>
<pre class="screen"><code class="prompt">$ </code><strong class="userinput"><code>cc $(pkg-config --libs --cflags libnm) -o hello-nm hello-nm.c</code></strong>
  <code class="prompt">$ </code><strong class="userinput"><code>./hello-nm</code></strong>
  NetworkManager version: 1.44.2

  <code class="prompt">$ </code></pre>
<p>
          Utilize the <code class="varname">PKG_CHECK_MODULES</code> macro to integrate with an
          autoconf-based build system. It's also recommended to use
          <code class="varname">NM_VERSION_MIN_REQUIRED</code> and <code class="varname">NM_VERSION_MAX_ALLOWED</code>
          macros to tell libnm headers which API version does your application need to work with.
          If you use them, the compiler will warn you when you use functionality that is not
          available in the versions you specified.
        </p>
<div class="informalexample">
  <table class="listing_frame" border="0" cellpadding="0" cellspacing="0">
    <tbody>
      <tr>
        <td class="listing_lines" align="right"><pre>1
2
3</pre></td>
        <td class="listing_code"><pre class="programlisting"><span class="n">PKG_CHECK_MODULES</span><span class="p">(</span><span class="n">LIBNM</span><span class="p">,</span><span class="w"> </span><span class="n">libnm</span><span class="w"> </span><span class="o">&gt;=</span><span class="w"> </span><span class="mf">1.8</span><span class="p">)</span>
<span class="n">LIBNM_CFLAGS</span><span class="o">=</span><span class="s">&quot;$LIBNM_CFLAGS -DNM_VERSION_MIN_REQUIRED=NM_VERSION_1_8&quot;</span>
<span class="n">LIBNM_CFLAGS</span><span class="o">=</span><span class="s">&quot;$LIBNM_CFLAGS -DNM_VERSION_MAX_ALLOWED=NM_VERSION_1_8&quot;</span></pre></td>
      </tr>
    </tbody>
  </table>
</div>

<p>
          You can use libnm from other languages than C with the use of GObject introspection.
          This includes Perl, Python, Javascript, Lua, Ruby and more. The example below shows what the
          typical libnm use in Python would look like.
        </p>
<div class="informalexample">
  <table class="listing_frame" border="0" cellpadding="0" cellspacing="0">
    <tbody>
      <tr>
        <td class="listing_lines" align="right"><pre>1
2
3
4
5
6</pre></td>
        <td class="listing_code"><pre class="programlisting"><span class="n">import</span><span class="w"> </span><span class="n">gi</span>
<span class="n">gi</span><span class="p">.</span><span class="n">require_version</span><span class="p">(</span><span class="err">&#39;</span><span class="n">NM</span><span class="err">&#39;</span><span class="p">,</span><span class="w"> </span><span class="err">&#39;</span><span class="mf">1.0</span><span class="err">&#39;</span><span class="p">)</span>
<span class="n">from</span><span class="w"> </span><span class="n">gi</span><span class="p">.</span><span class="n">repository</span><span class="w"> </span><span class="n">import</span><span class="w"> </span><span class="n">NM</span>

<span class="n">client</span><span class="w"> </span><span class="o">=</span><span class="w"> </span><span class="n">NM</span><span class="p">.</span><span class="n">Client</span><span class="p">.</span><span class="n">new</span><span class="p">(</span><span class="n">None</span><span class="p">)</span>
<span class="n">print</span><span class="w"> </span><span class="p">(</span><span class="s">&quot;NetworkManager version &quot;</span><span class="w"> </span><span class="o">+</span><span class="w"> </span><span class="n">client</span><span class="p">.</span><span class="n">get_version</span><span class="p">())</span></pre></td>
      </tr>
    </tbody>
  </table>
</div>

<p>
          There's <a class="ulink" href="https://lazka.github.io/pgi-docs/#NM-1.0" target="_top">NM-1.0 Python API Reference</a>
          maintained a third party that is generated from the introspection metadata.
        </p>
<p>
          In general, the C API documentation applies to the use GObject introspection
          from other languages, with the calling convention respecting the language's
          customs. Consult the source tree for
          <a class="ulink" href="https://gitlab.freedesktop.org/NetworkManager/NetworkManager/tree/main/examples" target="_top">some examples</a>.
        </p>
</div>
<div class="simplesect">
<div class="titlepage"><div><div><h3 class="title">
<a name="sync-api"></a>Synchronous API in libnm</h3></div></div></div>
<p>
          Libnm contains some synchronous API. This API basically makes a blocking
          D-Bus call (g_dbus_connection_call_sync()) and is now deprecated.
        </p>
<p>
          Note that D-Bus is fundamentally asynchronous. Doing blocking calls
          on top of D-Bus is odd, especially for libnm's NMClient. That is because
          NMClient essentially is a client-side cache of the objects of the D-Bus
          interface. This cache should be filled exclusively by (asynchronous) D-Bus
          events. So, making a blocking D-Bus call means to wait for a response and
          return it, while queuing everything that happens in between. Basically,
          there are three options how a synchronous API on NMClient could behave:
          </p>
<div class="orderedlist"><ol class="orderedlist" type="1">
<li class="listitem"><p>
                The call basically calls g_dbus_connection_call_sync(). This means
                that libnm sends a D-Bus request via GDBusConnection, and blockingly
                waits for the response. All D-Bus messages that get received in the
                meantime are queued in the GMainContext that belongs to NMClient.
                That means, none of these D-Bus events are processed until we
                iterate the GMainContext after the call returns. The effect is,
                that NMClient (and all cached objects in there) are unaffected by
                the D-Bus request.
                Most of the synchronous API calls in libnm are of this kind.
                The problem is that the strict ordering of D-Bus events gets
                violated.
                For some API this is not an immediate problem. Take for example
                nm_device_wifi_request_scan(). The call merely blockingly tells
                NetworkManager to start scanning, but since NetworkManager's D-Bus
                API does not directly expose any state that tells whether we are
                currently scanning, this out of order processing of the D-Bus
                request is a small issue.
                The problem is more obvious for nm_client_networking_set_enabled().
                After calling it, NM_CLIENT_NETWORKING_ENABLED is still unaffected
                and unchanged, because the PropertiesChanged signal from D-Bus
                is not yet processed.
                This means, while you make such a blocking call, NMClient's state
                does not change. But usually you perform the synchronous call
                to change some state. In this form, the blocking call is not useful,
                because NMClient only changes the state after iterating the GMainContext,
                and not after the blocking call returns.
              </p></li>
<li class="listitem"><p>
                Like 1), but after making the blocking g_dbus_connection_call_sync(),
                update the NMClient cache artificially. This is what
                nm_manager_check_connectivity() does, to "fix" bgo#784629.
                This also has the problem of out-of-order events, but it kinda
                solves the problem of not changing the state during the blocking
                call. But it does so by hacking the state of the cache. I think
                this is really wrong because the state should only be updated from
                the ordered stream of D-Bus messages. When libnm decides to modify
                the state, there are already D-Bus messages queued that affect this
                very state.
              </p></li>
<li class="listitem"><p>
                Instead of calling g_dbus_connection_call_sync(), use the
                asynchronous g_dbus_connection_call(). If we would use a separate
                GMainContext for all D-Bus related calls, we could ensure that
                while we block for the response, we iterate the internal main context.
                This might be nice, because all events are processed in order and
                after the blocking call returns, the NMClient state is up to date.
                The are problems however: current blocking API does not do this,
                so it's a significant change in behavior. Also, it might be
                unexpected to the user that during the blocking call the entire
                content of NMClient's cache might change and all pointers to the
                cache might be invalidated. Also, of course NMClient would invoke
                signals for all the changes that happen.
                Another problem is that this would be more effort to implement
                and it involves a small performance overhead for all D-Bus related
                calls (because we have to serialize all events in an internal
                GMainContext first and then invoke them on the caller's context).
                Also, if the users wants this, they could implement it themself
                using their own extra GMainContext and the asynchronous API.
              </p></li>
</ol></div>
<p>

          See also <a class="ulink" href="https://smcv.pseudorandom.co.uk/2008/11/nonblocking/" target="_top">this blog</a>
          for why blocking calls are wrong.
        </p>
<p>
          All possible behaviors for synchronous API have severe behavioural
          issues and thus such API is deprecated. Note that "deprecated" here does not
          mean that the API is going to be removed. Libnm does not break API. The
          user may:

          </p>
<div class="itemizedlist"><ul class="itemizedlist" style="list-style-type: disc; ">
<li class="listitem"><p>
                Continue to use this API. It's deprecated, awkward and discouraged,
                but if it works for you, that's fine.
              </p></li>
<li class="listitem"><p>
                Use asynchronous API. That's the only sensible way to use D-Bus.
                If libnm lacks a certain asynchronous counterpart, it should be
                added.
              </p></li>
<li class="listitem"><p>
                Use GDBusConnection directly. There really isn't anything wrong
                with D-Bus or GDBusConnection. This deprecated API is just a wrapper
                around g_dbus_connection_call_sync(). You may call it directly
                without feeling dirty.
              </p></li>
</ul></div>
<p>
        </p>
</div>
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